Why is one 340 connecting rod bronze color?

Crank Works - CWI Pro Rods

Heat Treatment
Our rods start with a copper plating of the non-critical areas. They are then heat treated to stringent specifications by a Nadcap, ISO, and AMS certified heat treating facility. This process substantially hardens the rods for increased strength. From there, the rods are stripped of their copper plating so that they can be properly shot-peened.